The present invention relates to the hardfacing of a surface, and, more particularly, to the deposition of a hardfacing deposit having a controlled, uniform maximum thickness and a smooth surface.
Hardfacings of relatively hard materials are positioned overlying the surfaces of relatively softer substrate materials in order to protect the softer materials against surface damage such as erosion, galling, and corrosion. The hardfacing material may be provided in any of several ways. In one approach, the hardfacing material is melted, contacted to the substrate, and allowed to solidify against the substrate. This technique is sometimes termed welding because the hardfacing deposit is joined to the substrate by solidification, but is distinct from the other type of welding wherein two solid pieces are joined together by a molten weldment.
In hardfacing by welding, the substrate is moved relative to the heat source and hardfacing material source. The hardfacing material initially forms a molten pool on the substrate surface, which thereafter solidifies as the heat source moves away from an area and heat is removed from the molten pool. The heat input from the heat source is usually adjusted so that the underlying substrate is melted to a shallow depth at its surface. Consequently, the molten hardfacing material and the melted region at the surface of the substrate material locally mix, with the result that, after solidification of the hardfacing, there is a good interfacial bond between the hardfacing deposit and the substrate.
While operable and widely used in industry, hardfacing by welding has drawbacks in some applications. Due to surface tension and other effects, upon solidification the hardfacing deposit is relatively irregular with a bumpy surface. For applications requiring a smooth surface, the hardfacing overlay must thereafter be ground or machined to make the surface smoother, an expensive and time-consuming operation. After grinding or machining, there may still be irregularities in the surface of the hardfacing overlay, between low spots and the machined or ground region. Surface cracks due to stress relief are sometimes found in the hardfacing, which can lead to penetration of erosive or corrosive agents into and through the hardfacing overlay. Such stress relief cracks are acceptable in limited numbers and particular orientations. However, it is preferred to reduce the incidence of cracking.
Additionally, it is difficult to apply a regular and uniform hardfacing deposit circumferentially onto a curved surface, such as the surface of a cylindrical pipe or shaft. The curved surface must be held horizontal, and the molten pool must be applied very near the top dead center of the horizontal region. Even then, because the solidification requires a period of time, the surface must be moved relatively slowly so that the still-molten pool does not run down the curved surface, resulting in wide variations in the thickness of the deposit. This slow movement reduces the production rate, with adverse effects on the economics of the hardfacing processing. And, even when the movement of the surface is relatively slow, the molten pool is often deformed and irregular, further increasing the tendency to surface irregularity of the final solidified deposit.
There is a need for an improved approach to hardfacing by welding that overcomes these drawbacks. The present invention fulfills this need, and further provides related advantages.